A fundamental challenge faced by designers of mass spectrometers is the efficient transport of ions from the ion source to the mass analyzer, particularly through atmospheric or low vacuum regions where ion motion is substantially influenced by interaction with background gas molecules. While electrostatic optics are commonly employed in these regions of commercially available mass spectrometer instruments for ion focusing, it is known that the effectiveness of such devices is limited due to the large numbers of collisions experienced by the ions. Consequently, ion transport losses tend to be high, which has a significant adverse impact on the instrument's overall sensitivity.
FIG. 1 is a simplified schematic diagram of a known mass spectrometer system 10. Referring to FIG. 1, an API source 12 housed in an ionization chamber 14 is connected to receive a liquid sample from an associated apparatus such as for instance a liquid chromatograph or syringe pump through a capillary 7. The API source 12 optionally is an electrospray ionization (ESI) source, a heated electrospray ionization (H-ESI) source, an atmospheric pressure chemical ionization (APCI) source, an atmospheric pressure matrix assisted laser desorption (MALDI) source, a photoionization source, or a source employing any other ionization technique that operates at pressures substantially above the operating pressure of mass analyzer 28 (e.g., from about 1 torr to about 2000 torr). The API source 12 forms charged particles 9 (either ions or charged droplets that may be desolvated so as to release ions) representative of the sample, which charged particles are subsequently transported from the API source 12 to the mass analyzer 28 in high-vacuum chamber 26 through at least one intermediate-vacuum chamber 18. In particular, the droplets or ions are entrained in a background gas and transported from the API source 12 through an ion transfer tube 16 that passes through a first partition element or wall 11 into an intermediate-vacuum chamber 18 which is maintained at a lower pressure than the pressure of the ionization chamber 14 but at a higher pressure than the pressure of the high-vacuum chamber 26. The ion transfer tube 16 may be physically coupled to a heating element or block 23 that provides heat to the gas and entrained particles in the ion transfer tube so as to aid in desolvation of charged droplets so as to thereby release free ions.
Due to the differences in pressure between the ionization chamber 14 and the intermediate-vacuum chamber 18 (FIG. 1), gases and entrained ions are caused to flow through ion transfer tube 16 into the intermediate-vacuum chamber 18. A plate or second partition element or wall 15 separates the intermediate-vacuum chamber 18 from either the high-vacuum chamber 26 or possibly a second intermediate-pressure region 25, which is maintained at a pressure that is lower than that of chamber 18 but higher than that of high-vacuum chamber 26. Vacuum port 13 is used for evacuation of the intermediate-vacuum chamber 18 by means of a mechanical pump or equivalent. Under typical operating conditions, the pressure within chamber 18 will be in the range of 1-50 Torr.
The analyte ions exit the outlet end of ion transfer tube 16 as a free jet expansion and travel through an ion channel 41 defined within the interior of ion transport device 40. As discussed in further detail in U.S. Pat. No. 7,781,728, the entire disclosure of which is incorporated herein by reference, radial confinement and focusing of ions within ion channel 41 are achieved by application of oscillatory voltages to apertured electrodes 44 of ion transport device 40. As is further discussed in U.S. Pat. No. 7,781,728, transport of ions along ion channel 41 to the device exit may be facilitated by generating a longitudinal DC field and/or by tailoring the flow of the background gas in which the ions are entrained. Ions leave the ion transport device 40 as a narrowly focused beam and are directed through aperture 22 of extraction lens 29 into the second intermediate pressure chamber 25.
Subsequently, the ions pass thereafter through ion optical elements 20, 31 and 24 and are delivered through aperture 27 to a mass analyzer 28 located within chamber 26. The ion optical assemblies or lenses 20, 24 may comprise transfer elements, such as, for instance a multipole ion guide. The mass analyzer 28 comprises one or more detectors 30 whose output can be displayed as a mass spectrum. As depicted in FIG. 1, the mass analyzer may take the form of a conventional two-dimensional quadruple trap having detectors 30. The mass analyzer 28 could alternatively comprise, a time of flight (TOE) mass analyzer, a Fourier transform mass analyzer, an ion trap, a magnetic sector mass analyzer or a hybrid mass analyzer. Chambers 25 and 26 may be evacuated to relatively low pressures by means of connection to ports of a turbo pump, as indicated by the arrows adjacent to vacuum port 17 and vacuum port 19. While ion transport device 40 is depicted as occupying a single chamber, alternative implementations may utilize an ion transport device that bridges two or more chambers or regions of successively reduced pressures.
The reader is referred to U.S. Pat. No. 7,781,728 for more details of the ion transport device 40. Briefly, the ion transport device 40 is formed from a plurality of generally planar electrodes 44 arranged in longitudinally spaced-apart relation (as used herein, the term “longitudinally” denotes the axis defined by the overall movement of ions along ion channel 41). Devices of this general construction are sometimes referred to in the mass spectrometry art as “stacked-ring” ion guides. Each electrode 44 is adapted with an aperture through which ions may pass. The apertures collectively define an ion channel 41, which may be straight or curved, depending on the lateral alignment of the apertures. To improve manufacturability and reduce cost, all of the electrodes 44 may have identically sized apertures. An oscillatory (e.g., radio-frequency) voltage source applies oscillatory voltages to electrodes 44 to thereby generate a field that radially confines ions within ion channel 41. In order to create a tapered field that focuses ions to a narrow beam near the exit of the ion transport device 40, the inter-electrode spacing or the oscillatory voltage amplitude is increased in the direction of ion travel.
The electrodes 44 of the ion transport device 40 may be divided into a plurality of first electrodes interleaved with a plurality of second electrodes, with the first electrodes receiving an oscillatory voltage that is opposite in phase with respect to the oscillatory voltage applied to the second electrodes. Further, a longitudinal DC field may be created within the ion channel 41 by providing a DC voltage source (not illustrated) that applies a set of DC voltages to electrodes 44 in order to assist in propelling ions through the ion transport device 40.
Ion funnel and stacked ring ion guide apparatuses all perform their intended functions adequately. Nonetheless, the use of these prior apparatuses does present some difficulties. First, it is difficult to completely block the “line-of-sight” through such apparatuses for the purpose of preventing neutral molecules from traveling to down-stream mass spectrometer components (including the detector) where they may cause undesirable contamination and spurious detector noise. Secondly, since such apparatuses comprise multiple electrodes, proper alignment of all the components is time consuming and subject to later disruption. Thirdly, for the same reason, such apparatuses are difficult to clean once they do become contaminated. Fourthly, the provision of many parallel electrode plates in these conventional apparatuses produces a naturally high capacitance which may draw high RF power.
Radio Frequency (RF) ion carpets are an alternative type of focusing ion guide. Such RF ion carpets have previously been used in high energy physics experiments, but have not been seen for analytical applications. For example, Takamine et al. (“Space-charge effects in the catcher gas cell of a RF ion guide,” Review of Scientific Instruments, 76[10], pp. 103503-103503-6, 2005) and Schwarz (“RF ion carpets: The electric field, the effective potential, operational parameters and an analysis of stability,” International Journal of Mass Spectrometry, 299[2-3], pp. 71-77, 2011) have described the use of ion carpets for the capture of high energy particles in high energy physics experiments. The ion carpet apparatus described by Takamine et al. consists of distinct inner and outer regions. The inner region includes 160 concentric ring electrodes to which both RF voltages and DC potentials are applied, the inner-region ring electrodes being included within a diameter of 110 mm and having equal widths of approximately 0.14 mm with 0.14 mm separations between electrodes. The outer region, occupying radii between 55-140 mm, consists of 85 additional equal-width concentric ring electrodes separated by 0.2 mm to which only DC potentials and no RF fields are applied, each such outer-region ring electrode being approximately 0.8 mm wide. Such ion carpet devices should be suitable for use in atmospheric pressure ionization sources used in mass spectrometers and ion mobility spectrometers, but, to date, have not been employed for these analytical applications.
The previously described RF ion carpets have been designed with equally sized and spaced electrodes. Such a design produces a restoring force near the surface of the RF ion carpet that is essentially constant. Such a design presents some problems for mass spectrometry and ion mobility spectrometry applications in that, for spectrometry applications, one would normally like a strong restoring force to initially capture and control the ions, and then a weak restoring force near the center, so that ions can be more easily extracted for transfer to optics further down stream in the instrument. Accordingly, there is a need in the art of analytical ion spectrometry—including mass spectrometry and ion mobility spectrometry—for anew type of ion guide apparatus that it is relatively simple to construct, is easy to clean, can easily provide a complete blockage of line of site, thereby reducing noise at the detector and eliminating contamination concerns, provides a large acceptance aperture than can easily collect ions from multiple sources and that has a lower capacitance and RF usage than an ion funnel or SRIG with an equivalent number of plates. The present invention addresses such a need.